The present invention relates to forming metal parts, and in particular, to an improved method and apparatus for simultaneously forming two parts from a pair of vertically oriented metal sheets while they are in a superplastic state.
For many years it has been known that certain metals, such as Titanium, as well as certain metal alloys, exhibit superplasticity within limited temperature ranges and strain rates. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency towards necking. Thus when in a superplastic condition, the metal or metal alloy exhibits low resistance to deformation and may be elongated with controlled thinning. This permits a sheet of such metal to be readily formed against dies to achieve desired shapes while maintaining a substantially uniform thickness in the finished part without any weak points. Superplastic forming (SPF) may be performed in conjunction with diffusion bonding. Diffusion bonding refers to metallurgical joining of surfaces of similar or dissimilar metals by holding them in physical contact and applying heat and pressure sufficient to cause commingling of the atoms at the junction. Further details of both SPF and diffusion bonding may be had by way of reference to U.S. Pat. No. 3,934,441 of Hamilton et al. entitled "Controlled Environment Superplastic Forming of Metals" and U.S. Pat. No. 3,927,817 of Hamilton et al. entitled "Method of Making Metallic Sandwich Structures."
U.S. Pat. No. 4,635,461 of Raymond entitled "Vertical Press" discloses a press for SPF or a combination of diffusion bonding and SPF, which may be utilized to make metallic sandwich structures. It has a pair of vertical ram assemblies, one of which is moved horizontally by four jack screws and the other one of which is moved horizontally by a pair of hydraulic cylinders. Four other hydraulic cylinders located at the corners of the latter ram assembly are used to align the same with respect to the tooling. A stack of three Titanium worksheets is formed by closing the ram assemblies to squeeze the sheets between metal tools which are backed by metal heating platens and ceramic insulator blocks. A seal in one of the tools is buried in the work sheets when the ram assemblies are fully closed so that gas pressure can be applied to effect diffusion bonding. Interlocking support members extend horizontally from the bottom of each of the ram assemblies for supporting the heavy metal tools, the worksheets, the heating platens and the insulator blocks. These elements all slide horizontally when the ram assemblies are separated. Tooling brackets may be attached to secure each insulator, heating platen and tooling die together.
The structure of the vertical press of U.S. Pat. No. 4,635,461 of Raymond has a tendency to sag. Considerable time is needed for the tooling to heat up in the press, which results in lower throughput. The use of both a fluid press and a screw press makes closure of the press complicated and time consuming. Significant temperature recovery time during the loading and unloading cycles also limits throughput. The metal heating platens tend to warp. The metal tooling is heavy and expensive. The vertical press of the Raymond patent is particularly adapted for forming a single three piece sandwich structure which requires that the edges of the sheets be firmly clamped via the imbedded seal in the tooling. This prevents inward slippage of the sheet edges. However, this approach is not compatible with tooling having relatively large horizontal recesses. With such tooling uniform thicknesses can only be achieved if the edges of the metal sheets can be vertically drawn in to accommodate substantial outward stretching of the sheets.